1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method and, more particularly, to image processing for quantizing, with a dither matrix, image data that has been corrected, to thus suppress density unevenness caused by variations in print characteristics between printing elements.
2. Description of the Related Art
The head shading (abbreviated as “HS”) technique disclosed in Japanese Patent Application Laid-open No. H10-13674 (1998) has been known as one example of the correction for suppressing an uneven density of the aforementioned type. This HS technique is adapted to perform correcting based on information on ink ejection characteristics (i.e., print characteristics) of each of nozzles serving as printing elements. For example, in response to information that amount of ink to be ejected by a certain nozzle is more than normal amount, image data is corrected so as to decrease a density indicated by the image data corresponding to the nozzle. In contrast, in response to information that amount of ink to be ejected by a certain nozzle is less than the normal amount, image data is corrected so as to increase a density indicated by the image data corresponding to the nozzle. In this manner, the number of ink dots to be eventually printed is decreased in the former case or increased in the latter case, so that the density of a print image formed based on the image data can become substantially even in each of the nozzles.
Such an HS technique detects density unevenness from the colorimetric result of a test pattern printed based on data on a predetermined density, and then, determines correction amount for HS processing. In a description below, out of these series of processing, processing up to correction amount determination is referred to as an HS analysis, and further, applying the correction amount resulting from the HS analysis to image data so as to correct the data is referred to as HS correction.
Dithering is one type of quantization for printing a test pattern in the HS analysis. In the case of the quantization by dithering, a value after the quantization is determined only based on a pixel value in input image data and a threshold arrangement on a dither matrix (i.e., a threshold arrangement pattern). As a consequence, dot arrangement on a test pattern to be printed is fixed according to the dither matrix.
In this case, the number of dots to be printed in the case of the quantization by dithering is varied according to positions of nozzles in a nozzle array direction corresponding to pixels to be quantized. For example, it is conceived that a test pattern image having a uniform pixel value is quantized with a dither matrix of dot concentration (fattening) type in a size of 8 pixels×8 pixels (64 gradations), as illustrated in FIG. 1. Here, the nozzle array direction is assumed to be an x direction.
FIGS. 2A and 2B illustrate quantization results with fattening type dither matrixes and the number of print dots in the nozzle array direction (i.e., the x direction), respectively, with respect to pieces of input image data having pixel values of 64, 128, and 192. In FIG. 2A, solid squares represent pixels to be printed with dots. Moreover, FIG. 2B illustrates the number of dots to be printed at pixel positions in the x direction. As illustrated in FIG. 2B, in the case where the pixel value of input image data is 192, the number of dots is 4 at pixel positions “1,” “4,” “5,” and “8” in the x direction whereas the number of dots is 8 at pixel positions “2,” “3,” “6,” and “7” in the x direction: namely, the number of dots to be printed is varied according to the pixel positions. In this manner, in the case where a uniform image consisting of pixels having the same pixel value is printed, the number of dots to be printed in the array direction of print elements (i.e., the x direction here) is unfavorably varied according to the pixel positions in the case of the quantization with the dither matrix. That is to say, the number of dots to be printed is varied at positions of pixels relative to a threshold arrangement pattern on the dither matrix. Alternatively, in an error diffusion method as another quantization technique, there is a dot delay region until errors are accumulated. However, when the number of dots to be printed at the pixel positions in the array direction of the print elements is counted while the dot delay region is bypassed, the number of dots is less varied than in the dither method. In other words, with the quantization by the dither method in printing the test pattern, the density of the test pattern to be printed becomes relatively largely uneven according to the pixel positions in the array direction of the print elements.
As described above, it is found that in the HS analysis, density unevenness including variations caused by the quantization in printing the test pattern is detected in addition to the variations in print characteristic such as ejection amount, and then, correction amount is determined.
Moreover, in the HS correction, the positions of the nozzles correspond to the correction positions, to which the correction amount determined by the HS analysis is applied. Specifically, image data on the pixels at positions corresponding to the positions of the nozzles in a print head is corrected in the HS correction, as illustrated in FIG. 3. Hereinafter, correspondence with reference to the positions of the nozzles will be referred to as “absolute position correspondence.” On the other hand, in printing an input image, the dither matrix to be used in the quantization is used in a manner corresponding to the pixel position of the input image, as illustrated in FIG. 4. Hereinafter, correspondence with reference to the positions of the pixels of the image will be referred to as “relative position correspondence.”
A printing apparatus generally copes with a plurality of widths of print mediums. In view of this, a test pattern is provided in such a manner as to print a maximum printable width, determine the correction amount with respect to all of nozzles, and thus, cope with any widths of print mediums. In the meantime, in printing an input image, a print medium having a width smaller than the maximum printable width may be used. Consequently, positions, to which dither matrixes are applied, with respect to positions of nozzles may be different between printing a test pattern and printing an input image, as illustrated in FIG. 4. As a consequence, the positions of pixels (i.e., nozzles) corresponding to correction amount that is determined by the HS analysis and includes quantization variations may be applied to image data at different pixel (i.e., nozzle) positions, that is, positions of pixels (i.e., nozzles) having different quantization variations during printing an input image. Consequently, the HS correction cannot be properly made, thereby causing the above-described density unevenness on a printout of the input image due to the quantization with the dither matrix. As described above, since the dither method causes larger density unevenness by the quantization than by, for example, the error diffusion method, the problem of the density unevenness becomes more serious in the case of the dither method used for the quantization.
Incidentally, as disclosed in Japanese Patent Application Laid-Open No. 2007-196472, there has been known a technique for modifying a dither matrix corresponding to each of nozzles according to density unevenness of each of the nozzles. This technique can correct density unevenness. With this technique, quantization also is performed in accordance with the absolute position correspondence. However, this technique relates to a print head having one nozzle array for each of colors at a print position in an x direction. Japanese Patent Application Laid Open No. 2007-196472 is silent about a print head having a multiple-array configuration consisting of a plurality of nozzles for colors. As described later, when nozzles are determined based on a quantization result by the dither method and a distribution pattern for multiple arrays (i.e., an array distribution pattern), an ejection rate between arrays is varied according to positions in an x direction. Ejection amount may be varied in nozzles at the same position in the x direction out of multiple-array nozzles due to causes from the viewpoint of fabrication or the like. Therefore, variations in ejection rate between the arrays of the multiple-array nozzles and variations in ejection amount are reflected on density unevenness in the x direction. Thus, the technique relevant to the one-array configuration disclosed in Japanese Patent Application Laid Open No. 2007-196472 cannot correct the density unevenness of a line head having the multiple-array configuration.